I. Field of the Invention
The disclosed embodiments relate to wireless communications. More particularly, the disclosed embodiments relate to a novel and improved method of forward link power control in a wireless communication system.
II. Background
Power control in a wireless communication system is advantageous for many reasons. For example, battery power conservation in a wireless communication device is optimized by controlling the transmitter output power such that the device does not transmit with excessive power. In addition, lowering the transmission output power decreases the interference to other wireless communication devices.
In certain wireless communication systems, such as a code division multiple access (CDMA) communication system, power control is absolutely essential. As those skilled in the art can appreciate, CDMA technology assigns statistically uncorrelated binary sequences, sometimes referred to as pseudo-random sequences, to each wireless communication device. The data signal from a CDMA device is modulated in a pseudo-random fashion using the pseudo-random sequences such that a receiver can demodulate using the same pseudo-random sequence to recover the data signal while any other signals are decoded as noise. Thus, an increase in the power of signals will also increase the noise at any given device.
If one CDMA device increased its transmission power, the signal for that particular device would increase, but at the cost of a decreased signal-to-noise ratio (SNR) for every other user. That is, an increased signal for one user results in increased noise for every other user and vice versa. To offset this decrease in SNR, each of the other users must increase their signal power, which in turn will increase the noise level for all users. It is therefore easy to appreciate the importance of each user providing the correct amount of signal power required and no more. As a result, it is essential to transmit at the lowest possible power level that will still allow acceptable quality of communication. In addition, the channel usually changes with time and the received power can change rapidly thus requiring a dynamic power control mechanism. An exemplary power control technique is described in U.S. Pat. No. 5,267,162 assigned to the assignee of the present invention and fully incorporated herein by reference.
Power control in a forward link (from a base transceiver subsystem to a wireless device) of a CDMA system is particularly difficult during handoff. In a “soft” handoff, the wireless device may be communicating with two or more base transceiver subsystems (BTS). If forward link power control is not adequately regulated, the BTSs can transmit at radically different power levels resulting in an overall system imbalance. A known contributor to unbalanced forward link transmit power is the current Or-Of-Down logic implementation, described herein below, used to control reverse link (from a wireless device to a base transceiver subsystem) power levels. Forward link power control commands transmitted on the reverse link cannot be properly received by a BTS if the reverse link transmit power is inadequate.
In CDMA soft handoff, different BTSs command the wireless device, or mobile unit, to raise or lower its power level depending on the quality of the signal received by each BTS while the mobile unit transmits one signal to multiple BTSs. Depending on the location and channel conditions of the BTSs, the different BTSs receive different SNRs or signal quality in general. Each BTS decodes received frames and decides if the frame is good or bad, depending on encoded error correction information. If the base station has multiple sectors, the signals from the different sectors are combined before the decoding. In general, each BTS is making frame quality decisions based on the level of reception quality, which is a product of the SNR, or level of interference. Each BTS measures SNR and decides if the received signal quality is acceptable. The BTS commands the mobile unit to transmit the power level desired by the BTS to maintain acceptable received signal quality. Each BTS has its own target level of desired quality set by a Selector system component typically located in a Base Station Controller (BSC).
The mobile unit combines the transmit power level commands it receives from all the BTSs involved in soft handoff using OR-of-down logic. OR-of-down logic is a logical ORing of the down commands, or instructions, from the BTSs to reduce transmit power. If one base station with good receive signal quality tells the mobile unit to go down in power, the mobile unit reduces power regardless of commands from other BTSs with poor received signal quality to increase transmit power. OR-of-down logic causes the mobile unit to transmit at the power level desired by the base station requesting the lowest power level. Generally, all the BTSs except one do not get a desired level of received signal quality because the mobile unit will continue to reduce transmit power until the last BTS stops commanding it to reduce power. The received signal quality of the other BTSs is typically not as good as the signal quality received by the BTS receiving the best signal quality. If the BTS received signal quality is not good enough, the BTS cannot correctly detect the reverse link (mobile unit to BTS) power control bits, which indicate to the BTS whether or not to increase or decrease its forward link transmit power level. A problem is created when incorrectly detected power control bits cause a BTS power level to fail to move up and down with the other BTSs involved in a soft handoff because the BTS transmit levels become divergent. Divergent power levels restrict the ability of the mobile unit to combine the BTS signals. Ideally, all the BTSs involved in a soft handoff must have a minimum received power control bit signal quality from the same mobile unit for a reliably balanced forward link.
Therefore, it can be appreciated that there is a significant need for a method that controls forward link power in a wireless communication system providing improved feedback quality so that every base station can listen to the same mobile unit in soft handoff equally well.